Drag and Drop onto a canvas that expands with things dropped into it

Hi, I have a control which is basically a canvas with a textbox and a border around it. I want to be able to drag one of these objects onto another one and have that one expand to encompass what was dropped into it (similar to the concept of "grouping" in say, microsoft word or powerpoint). The thing is that I want the canvas/control to expand around the other one and automatically "group" it into itself so when i drag the outer one around the screen everything inside of it follows.

Now I've been able to get drag functionality to work with these controls, but not the drop and expand stuff. I've also thought that maybe a grid would work for this instead of a canvas, but as I'm kind of a WPF noob, I don't know much about how grids work.

Can anyone help me with this? And would you suggest switching my controls so they use a grid instead of a canvas?

Tags: C#, XAML, WPF